Indoor localization technology aims to automatically locate devices or people in indoor environments, such as inside buildings. In many indoor scenarios, the Global Positioning System (GPS) is typically inaccessible due to blockage of satellite signals. Consequently, other techniques have been developed that use a network of interconnected devices to track the location of a target. For example, techniques that use time-of-arrival (TOA) estimations have been developed. These techniques obtain signal flight distance as a pseudorange, and trilateration methods utilize these pseudoranges to calculate location information.
Mainly, two categories of approaches have been proposed to solve this problem. The first type of solution utilizes Impulse Radio Ultra-Wideband (IR-UWB) technique for indoor TOA-based ranging, as ranging precision typically increases with the bandwidth of the operating signal. Using UWB signals has attracted significant research interests and become a standard as IEEE 802.15.4a. The second type of solution utilizes ultrasound signals to perform ranging. Compared with electromagnetic signals used in a IR-UWB device, aerial acoustic signals are more pervasive and can typically achieve ranging accuracy with lower hardware cost. Due to slower transmission speed of acoustic signals, even a several kHz signal bandwidth can result in centimeter-level ranging accuracy.